linux-kernel - Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event

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Message-ID: <20140314141300.GG3793@tassilo.jf.intel.com>
Date:	Fri, 14 Mar 2014 07:13:00 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event

> > The other big problem is scalability. Even if it was somehow possible
> > to make this scheme work the IPIs for flushing would kill performance 
> > on any multi threaded client.  Given perf is not multi-threaded today, but
> > it doesn't seem a good idea to design the interface assuming no client ever
> > will be.
> 
> Well any mmap()ed interface that wants to swap buffers will have this
> same problem.

There's no need to swap buffers in a sane design. The perf ring buffer
or the ftrace buffer don't need this. There's no need for a PT buffer
to do so either. 

> 
> You can restrict the TLB flushing to the threads that poll() on the
> relevant events. This just means other threads will see old/partial
> data, but that shouldn't be a problem as they shouldn't be looking in
> the first place.

Then we get incoherent processes. You're not serious about that are you?

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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